Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Multiple wavelengths may be transmitted along the same optic fiber. The totality of multiple combined wavelengths comprises a single transmitted signal. A crucial feature of a fiber optic network is the separation of the optical signal into its component wavelengths, or "channels", typically by a wavelength division multiplexer. This separation must occur in order for the exchange of wavelengths between signals on "loops" within networks to occur. The exchange occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths.
In this specification, individual information-carrying lights are referred to as "channels". The totality of multiple combined channels in a wavelength-division multiplexed optical fiber, optical line or optical system, where each channel is of a different wavelength range, is referred to as an "optical signal". The term "wavelength" is used synonymously with the term "channel". Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength.
Add/drop systems exist at the connector points for the management of the channel exchanges. The exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network. In other words, each signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop. The adding and dropping of channels always occur together.
FIG. 1 illustrates a simplified optical network 100. A fiber optic network 100 could comprise a main loop 150 which connects primary locations, such as San Francisco and New York. In-between the primary locations is a local loop 110 which connects with loop 150 at connector point 140. Thus, if local loop 110 is Sacramento, wavelengths at San Francisco are multiplexed into an optical signal which will travel from San Francisco, add and drop channels with Sacramento's signal at connector point 140, and the new signal will travel forward to New York. Within loop 110, optical signals would be transmitted to various locations within its loop, servicing the Sacramento area. Local receivers (not shown) would reside at various points within the local loop 110 to convert the optical signals into the electrical signals in the appropriate protocol format.
The separation of an optical signal into its component channels is typically performed by a dense wavelength division multiplexer. FIG. 2 illustrates add/drop systems 200 and 210 with dense wavelength division multiplexers 220 and 230. An optical signal from Loop 110 (.lambda..sub.1 -.lambda..sub.n) enters its add/drop system 200 at node A (240). The signal is separated into its component channels by the dense wavelength division multiplexer 220. Each channel is then outputted to its own path 250-1 through 250-n. For example, .lambda..sub.1 would travel along path 250-1, .lambda..sub.2 would travel along path 250-2, etc. In the same manner, the signal from Loop 150 (.lambda..sub.1 '-.lambda..sub.n ') enters its add/drop system 210 via node C (270). The signal is separated into its component channels by the wavelength division multiplexer 230. Each channel is then outputted via its own path 280-1 through 280-n. For example, .lambda..sub.1 ' would travel along path 280-1, .lambda..sub.2 ' would travel along path 280-2, etc.
In the performance of an add/drop function, for example, .lambda..sub.1 is transferred from path 250-1 to path 280-1. It is combined with the others of Loop 150's channels into a single new optical signal by the dense wavelength division multiplexer 230. The new signal is then returned to Loop 150 via node D (290). At the same time, .lambda..sub.1 ' is transferred to path 250-1 from 280-1. It is combined with the others of Loop 110's channels into a single optical signal by the dense wavelength division multiplexer 220. This new signal is then returned to Loop 110 via node B (260). In this manner, from Loop 110's frame of reference, channel .lambda..sub.1 of its own signal is dropped to Loop 150 while channel .lambda..sub.1 ' of the signal from Loop 150 is added to form part of its new signal. The opposite is true from Loop 150's frame of reference. This is the add/drop function.
Conventional methods used by wavelength division multiplexers in separating an optical signal into its component channels include the use of filters and fiber gratings as separators. A "separator," as the term is used in this specification, is an integrated collection of optical components functioning as a unit which separates one or more channels from an optical signal. Filters allow a target channel to pass through while redirecting all other channels. Fiber gratings target a channel to be reflected while all other channels pass through. Both filters and fiber gratings are well known in the art and will not be discussed in further detail here.
A problem with the conventional separators is the precision required of a device for transmitting a signal into an optic fiber. A signal entering a wavelength division multiplexer must conform to a set of very narrow pass bands. FIG. 3 shows a sample spectrum curve 310 composed of numerous channels as they enters a dense wavelength division multiplexer. The pass bands 320 of the channels are very narrow. Ideally, the curve would be a square wave. A narrow pass band is problematic because, due to the physical limitations and temperature sensitivity of signal source devices, they never emit light exactly at the center wavelengths of the pass bands of an optical filter. The difference between the actual wavelength and the center of the pass band is called the "offset." The amount of offset or change in offset ("drift") ideally should not be larger than the width of the pass band. Otherwise, crosstalk between channels will be too large. Crosstalk occurs when one channel or part of a channel appears as noise on another channel adjacent to it. Since the signals resulting from the conventional wavelength division multiplexer configurations have narrow pass bands, the signal source devices ("transmitters"), such as lasers or the like, must be of a high precision so that drift is limited to the width of the pass bands. This high precision is difficult to accomplish. Signal source devices of high precision are available but are very expensive. Also, the signal source devices must be aligned individually for each separator, which is time intensive.
Therefore, there exists a need for a wavelength division multiplexer with a method of separation which has a greater tolerance for drift and is easier to align. This method should also be cost effective to implement. The present invention addresses such a need.